The need for off-press colour proofing materials in the graphic arts industry as an agreed standard between printer and customer, to define the finished look of printed material and to set up printing press conditions is well known. There are many proofing systems commercially available which fall into two general categories; critical or non-critical, depending on whether high or lower quality colour images are produced.
One type of proofing system is the wet-developed overlay proofing system. In overlay systems the colour layer is mounted on a transparent plastic support. Each colour sheet is exposed and wet-developed. Four single colour images are obtained, each on its support; cyan, magenta, yellow and black. These are then overlaid in register, giving a full colour image with four relatively thick plastic support sheets intervening. Proofs composed in this manner must be non-critical for several reasons including internal reflection and a high optical dot-gain from the four intervening plastic sheets. Examples of commercially available wet-developed overlay proofs include 3M Color Key, Enco NAPS and PAPS, Kodak Double Check and Fuji Color Art Overlay.
Wet-developed single-sheet proofing systems are known including those available commercially under the trade names 3M Matchprint, Agfaproof, Fuji Color Art, and Polaroid SpectraProof. Proofing materials for such proofing systems are usually composed of cyan, yellow, magenta and black colour sheets. In the Matchprint system the first colour sheet, often cyan, is laminated onto Matchprint base receptor (white card) and its transparent colourless support sheet removed. The colour layer on the base is contact exposed through a corresponding graphic arts film colour separation. The base and colour layer are then passed through a wet-developer machine, after which the cyan image component is visible. The next colour sheet is then laminated onto the first image, support removed, exposure carried out with the appropriate colour separation in register with the underlying image, and wet developed. The laminate-expose-develop cycle is repeated for the two remaining colour components and a full colour high quality surprint (integral) proof produced. The Fuji Color Art system employs similar process steps, but in the sequence: exposure, wet development and lamination. Lamination of the second and subsequent sheets must be carried out in register with the first image.
These and other wet-development systems involve a colour layer which, in a development liquid, becomes less soluble (negative-acting) or more soluble (positive-acting) when light exposed. During development the more soluble portions of the coloured material are washed away in the developing liquid, revealing the image. A typical construction is disclosed in U.S. Pat. No. 3,671,236 and comprises a carrier sheet in releasable attachment to a photosensitive colour layer, over which is coated in turn a water-insoluble colourless transparent barrier layer and a pressure-sensitive adhesive layer. The barrier layer is designed to withstand attack by aqueous-organic developer solutions, and polyacrylate resins are exemplified as suitable barrier materials. The barrier layer is designed to withstand attack by aqueous-organic developer solutions, and polyacrylate resins are exemplified as suitable barrier materials. Some systems are based on conventional silver halide image development, but currently these also require wet chemical development.
Despite the success of wet-developed contact proofing systems, they suffer from two major disadvantages which are becoming increasingly important. Developing machines normally require a continuous supply of fresh developer to replace reacted developer and maintain consistency, so therefore drums of developer must be regularly delivered. There is also a continuous discharge of chemical waste during processing into the drainage system which is undesirable in view of the increasing importance of environmental acceptability in product use. For these reasons, there has been an increasing interest in dry-developed contact proofing.
Potential methods of dry-development include thermal dye transfer, thermal mass transfer, peel-apart methods, and toner application methods.
There are commercially available proofing systems based on a colourless coated surface which after imagewise exposure carries a tacky image. Toner powder (Cromalin proofing system) or a coloured donor coating (Eurosprint proofing system) (both available from DuPont) is applied to this receptor sheet, transferring colour to the tacky areas.
One of the simplest methods of dry process proofing is a peel-apart system. Potential advantages include lower wet waste production, faster throughput and reduced machinery requirements. For these reasons the interest in obtaining a dry developed colour proofing system has increased.
Peel-apart processing constitutes a simple method of dry developing an essentially one colour image. In its simplest form, two co-extensive sheets, e.g. of polyester, sandwich a coloured layer. The construction is exposed imagewise through whichever sheet is considered the cover and peeled apart by gaining a purchase on the coversheet (or base) and peeling the coversheet from the base. The colour layer breaks imagewise with a positive image of the exposure mask separating on the cover and the corresponding negative image on the base or vice-versa.
Generally, the basis of this type of peel-apart system has been photopolymerisation chemistry in the colour layer which is imagewise photohardened. This is one way of supplying a photochemically induced change in the mechanical properties of the colour layer which is needed for an imagewise separation.
Cromacheck, an overlay peel-apart system commercially available from DuPont consists essentially of the above system. This, and related systems are disclosed in U.S. Pat. Nos. 3,754,920, 4,282,308, 4,247,619, 4,489,153 and 4,316,951. Each colour sheet is imagewise exposed through the appropriate colour separation and peeled to give cyan, magenta, yellow and black images on separate supports which are then overlaid in register to give a non-critical proof. The peel-apart elements generally comprise (in order) a strippable cover sheet, a photoadherent layer, a non-photosensitive tacky contiguous layer and a substrate. The photoadherent layer may be coloured, in which case imagewise exposure and peel-apart reveals the image immediately, or it may be colourless, in which case the image is revealed by applying toner powder to the areas of the tacky contiguous layer that are bared by the peel-apart process, as disclosed in U.S. Pat. No. 4,247,619. The tacky contiguous layer generally comprises a soft, elastomeric resinuous material.
The disadvantages of the overlay scheme have been outlined above and a high quality critical proof cannot be made because of the intervening transparent plastic supports.
The peel-apart overlay method involves two steps per colour (exposure and peel-apart) plus the final assembly of the four colours in register to give the full colour image--a total of nine steps. Although the image thus produced is non-critical it has the advantage that the assembly method is simple and quick. In proofing it is highly desirable that the minimum number of steps be involved in making the proof so that maximum productivity may be obtained. One of the two pieces of polyester is discarded per colour layer. It is also desirable to keep polyester usage and waste to a minimum. Polyester is an expensive material component in products of this type.
Peel-apart development has also been applied to the production of surprint (integral) proofs.
For example, EP-A-0385466 discloses a method which involves a peel, laminate, expose and imaging peel-apart step for each colour sheet but no separate final assembly--a total of sixteen steps. Two pieces of polyester are discarded per colour layer. Each sheet comprises a support bearing an elastomeric layer, colour photolayer, release layer and cover. The support is removed from a first colour sheet and the elastomeric layer laminated to a base. After exposure the cover is removed as the peel-apart developing step. Thereafter, each colour sheet is applied, exposed and developed sequentially.
U.S. Pat. No. 4,987,051 discloses a system in which each colour sheet comprises a support bearing an elastomer layer, colour photopolymer layer and cover. For integral proof assembly, each colour layer support is removed and then lamination carried out. The cover is then removed, exposure is carried out, the cover relaminated, a blanket exposure carried out and then the cover is peeled once more to develop the colour image. This gives an integral proof but involves a total of twenty-eight steps, seven per colour layer. A similar scheme is disclosed in EP-A-0514186.
A similar scheme which relies on the photogeneration of an inhibitor in exposed areas also gives a negative-acting surprint proof as disclosed in U.S. Pat. No. 4,923,780. U.S. Pat. No. 4,489,154 discloses a system in which each colour sheet comprises a support, elastomer layer, colour layer and cover.
The process involves exposure of each sheet, peel-apart, lamination in register onto a receptor and a further peel-off--a total of sixteen steps per proof with two pieces of polyester discarded per colour layer.
U.S. Pat. No. 4,895,787 discloses a system in which each colour sheet comprises a cover in the form of an adhesion promoted substrate bearing a colour layer and hot melt adhesive layer. The sheet may be exposed before or after lamination to a receptor and thereafter the cover peeled-apart. Very similar materials are disclosed in EP-A-0525624.
This method is perhaps the simplest, involving lamination, exposure through a cover sheet and peel-apart--a total of twelve steps per proof. Only one piece of polyester is used and then discarded per colour layer. However, the number of adhesives that can be used is limited because the adhesive must (a) be coated out of a solvent that does not disrupt the underlying colour layer or (b) be hot-melt coated or laminated or (c) be coated as an aqueous emulsion which must transparentise when dried. Most of the examples in the patent describe application of the adhesive by lamination rather than coating. If this is the procedure used, then more polyester supports may be required for the adhesive donor sheets. Each of the four colour sheets requires a donor sheet for adhesive application composed of a polyester support and the hot-melt adhesive layer. This could be part of the proof assembly process as used by the customer, or done during the manufacturing stage. The total effective number of supports used is thus eight. One other potential problem with this type of construction where the adhesive layer is in direct contact with the colour layer is staining, caused by migration of coloured materials into the adhesive layer during manufacture or assembly.
U.S. Patent Nos 4,910,120 and 4,963,462 disclose more complicated schemes involving twenty steps. Two pieces of polyester are discarded per colour layer. However, the methods have the potential for choice of the receptor base, e.g. printing stock on which to view the proof. Other proofing schemes are disclosed in European Patent Publication Nos. 0465087 and 0365362.
The aforementioned methods for integral proofing disclosed in the art are summarised in the following table:
______________________________________ Patent Number Number of Image Publication of Discarded (relative Number Steps Supports to mask) ______________________________________ EP 0385466 16 8 Negative US 4,987,051 28 8 Negative US 4,489,154 16 8 Negative US 4,923,780 16 4 Negative US 4,895,787 12 4 Positive US 4,963,462 20 8 Positive EP 0465087 20 8 Positive US 4,910,120 20 8 Positive EP 0365362 20 8 Negative ______________________________________
The type of image (negative or positive) refers to the polarity of the final image on the base in relation to the image on the transparency through which the photosensitive media is exposed. In the case of a negative-acting system, the photosensitive layer prior to exposure shows a greater adherence to the strippable substrate than to the base, but after exposure, adheres preferentially to the base.
As can be seen from the table, U.S. Pat. No. 4,895,787 allows for the minimum waste of polyester and the minimum number of steps. This gives a positive-acting surprint proofing system.
More complex peel-apart elements suitable for the construction of integral proofs are disclosed in International Patent Application No. WO92/15920. These comprise (in order) a strippable coversheet, a photohardenable layer, a contiguous layer, a non-elastomeric isolation layer, an adhesive layer and a support. In use, the support is removed and the element laminated to a base via the adhesive layer. Thereafter, the element is imagewise exposed through a coversheet which is then peeled, removing with it areas of the photohardenable layer in accordance with the imagewise exposure. In embodiments in which the photohardenable layer is coloured, this reveals an image. In embodiments in which the photohardenable layer is not coloured, it is necessary to apply toner to the bared areas of the contiguous layer. Depending on its formulation, the photohardenable layer may adhere preferentially to the cover sheet or to the contiguous layer in exposed areas, and so the system may be adapted to provide either positive or negative proofs.
The contiguous layer comprises a soft, deformable material such as a rubber or other elastomeric polymer. The isolation layer comprises a non-elastomeric polymer that is less deformable than the polymers contained in the contiguous and adhesive layers. Exemplified materials are polymers and copolymers of vinyl acetate and acrylic polymers. The media are said to give improved quality images with reduced dot gain, but the exact function of the isolation layer is not described.
It can be seen in many of the above schemes that in order to obtain a left to right reading image, exposure must be through a relatively thick support/cover layer. It is well known that exposure through a thickness of a transparent substrate detrimentally affects image resolution or dot size by light undercutting the shielding areas of the exposure mask. This can be avoided by removing the cover and exposing, then relaminating the cover for the peel-apart step. However, this introduces additional time consuming steps lowering productivity and increasing the complexity of proof assembly for the end user.
It is well known that reflection of radiation from the white supporting base during the imaging exposure, (known as halation) detrimentally affects image quality. This, and the undercutting, reduce image quality. However, the reflected component can be reduced by incorporating an actinic radiation absorbing antihalation material in a layer opposite to the exposing side of the photosensitive layer. This improves image quality in the resultant peel-apart image. The incorporation of such a material is difficult in the simplest of the methods in the art. Such a material might be included in the adhesive layer, but the number of solvents usable is limited since this is coated directly onto the solvent sensitive photosensitive layer. Use of a water based adhesive even further limits the number of radiation absorbing compounds that might be used.